1Memory Resource Controller
2 3NOTE: The Memory Resource Controller has been generically been referred
4to as the memory controller in this document. Do not confuse memory controller
5used here with the memory controller that is used in hardware.
6 7Salient features
8 9a. Enable control of both RSS (mapped) and Page Cache (unmapped) pages
10b. The infrastructure allows easy addition of other types of memory to control
11c. Provides *zero overhead* for non memory controller users
12d. Provides a double LRU: global memory pressure causes reclaim from the
13 global LRU; a cgroup on hitting a limit, reclaims from the per
14 cgroup LRU
15 16NOTE: Swap Cache (unmapped) is not accounted now.
17 18Benefits and Purpose of the memory controller
19 20The memory controller isolates the memory behaviour of a group of tasks
21from the rest of the system. The article on LWN [12] mentions some probable
22uses of the memory controller. The memory controller can be used to
23 24a. Isolate an application or a group of applications
25 Memory hungry applications can be isolated and limited to a smaller
26 amount of memory.
27b. Create a cgroup with limited amount of memory, this can be used
28 as a good alternative to booting with mem=XXXX.
29c. Virtualization solutions can control the amount of memory they want
30 to assign to a virtual machine instance.
31d. A CD/DVD burner could control the amount of memory used by the
32 rest of the system to ensure that burning does not fail due to lack
33 of available memory.
34e. There are several other use cases, find one or use the controller just
35 for fun (to learn and hack on the VM subsystem).
36 371. History
38 39The memory controller has a long history. A request for comments for the memory
40controller was posted by Balbir Singh [1]. At the time the RFC was posted
41there were several implementations for memory control. The goal of the
42RFC was to build consensus and agreement for the minimal features required
43for memory control. The first RSS controller was posted by Balbir Singh[2]
44in Feb 2007. Pavel Emelianov [3][4][5] has since posted three versions of the
45RSS controller. At OLS, at the resource management BoF, everyone suggested
46that we handle both page cache and RSS together. Another request was raised
47to allow user space handling of OOM. The current memory controller is
48at version 6; it combines both mapped (RSS) and unmapped Page
49Cache Control [11].
50 512. Memory Control
52 53Memory is a unique resource in the sense that it is present in a limited
54amount. If a task requires a lot of CPU processing, the task can spread
55its processing over a period of hours, days, months or years, but with
56memory, the same physical memory needs to be reused to accomplish the task.
57 58The memory controller implementation has been divided into phases. These
59are:
60 611. Memory controller
622. mlock(2) controller
633. Kernel user memory accounting and slab control
644. user mappings length controller
65 66The memory controller is the first controller developed.
67 682.1. Design
69 70The core of the design is a counter called the res_counter. The res_counter
71tracks the current memory usage and limit of the group of processes associated
72with the controller. Each cgroup has a memory controller specific data
73structure (mem_cgroup) associated with it.
74 752.2. Accounting
76 77 +--------------------+
78 | mem_cgroup |
79 | (res_counter) |
80 +--------------------+
81 / ^ \
82 / | \
83 +---------------+ | +---------------+
84 | mm_struct | |.... | mm_struct |
85 | | | | |
86 +---------------+ | +---------------+
87 |
88 + --------------+
89 |
90 +---------------+ +------+--------+
91 | page +----------> page_cgroup|
92 | | | |
93 +---------------+ +---------------+
94 95 (Figure 1: Hierarchy of Accounting)
96 97 98Figure 1 shows the important aspects of the controller
99 1001. Accounting happens per cgroup
1012. Each mm_struct knows about which cgroup it belongs to
1023. Each page has a pointer to the page_cgroup, which in turn knows the
103 cgroup it belongs to
104 105The accounting is done as follows: mem_cgroup_charge() is invoked to setup
106the necessary data structures and check if the cgroup that is being charged
107is over its limit. If it is then reclaim is invoked on the cgroup.
108More details can be found in the reclaim section of this document.
109If everything goes well, a page meta-data-structure called page_cgroup is
110allocated and associated with the page. This routine also adds the page to
111the per cgroup LRU.
112 1132.2.1 Accounting details
114 115All mapped anon pages (RSS) and cache pages (Page Cache) are accounted.
116(some pages which never be reclaimable and will not be on global LRU
117 are not accounted. we just accounts pages under usual vm management.)
118 119RSS pages are accounted at page_fault unless they've already been accounted
120for earlier. A file page will be accounted for as Page Cache when it's
121inserted into inode (radix-tree). While it's mapped into the page tables of
122processes, duplicate accounting is carefully avoided.
123 124A RSS page is unaccounted when it's fully unmapped. A PageCache page is
125unaccounted when it's removed from radix-tree.
126 127At page migration, accounting information is kept.
128 129Note: we just account pages-on-lru because our purpose is to control amount
130of used pages. not-on-lru pages are tend to be out-of-control from vm view.
131 1322.3 Shared Page Accounting
133 134Shared pages are accounted on the basis of the first touch approach. The
135cgroup that first touches a page is accounted for the page. The principle
136behind this approach is that a cgroup that aggressively uses a shared
137page will eventually get charged for it (once it is uncharged from
138the cgroup that brought it in -- this will happen on memory pressure).
139 140Exception: If CONFIG_CGROUP_CGROUP_MEM_RES_CTLR_SWAP is not used..
141When you do swapoff and make swapped-out pages of shmem(tmpfs) to
142be backed into memory in force, charges for pages are accounted against the
143caller of swapoff rather than the users of shmem.
144 145 1462.4 Swap Extension (CONFIG_CGROUP_MEM_RES_CTLR_SWAP)
147Swap Extension allows you to record charge for swap. A swapped-in page is
148charged back to original page allocator if possible.
149 150When swap is accounted, following files are added.
151 - memory.memsw.usage_in_bytes.
152 - memory.memsw.limit_in_bytes.
153 154usage of mem+swap is limited by memsw.limit_in_bytes.
155 156Note: why 'mem+swap' rather than swap.
157The global LRU(kswapd) can swap out arbitrary pages. Swap-out means
158to move account from memory to swap...there is no change in usage of
159mem+swap.
160 161In other words, when we want to limit the usage of swap without affecting
162global LRU, mem+swap limit is better than just limiting swap from OS point
163of view.
164 1652.5 Reclaim
166 167Each cgroup maintains a per cgroup LRU that consists of an active
168and inactive list. When a cgroup goes over its limit, we first try
169to reclaim memory from the cgroup so as to make space for the new
170pages that the cgroup has touched. If the reclaim is unsuccessful,
171an OOM routine is invoked to select and kill the bulkiest task in the
172cgroup.
173 174The reclaim algorithm has not been modified for cgroups, except that
175pages that are selected for reclaiming come from the per cgroup LRU
176list.
177 1782. Locking
179 180The memory controller uses the following hierarchy
181 1821. zone->lru_lock is used for selecting pages to be isolated
1832. mem->per_zone->lru_lock protects the per cgroup LRU (per zone)
1843. lock_page_cgroup() is used to protect page->page_cgroup
185 1863. User Interface
187 1880. Configuration
189 190a. Enable CONFIG_CGROUPS
191b. Enable CONFIG_RESOURCE_COUNTERS
192c. Enable CONFIG_CGROUP_MEM_RES_CTLR
193 1941. Prepare the cgroups
195# mkdir -p /cgroups
196# mount -t cgroup none /cgroups -o memory
197 1982. Make the new group and move bash into it
199# mkdir /cgroups/0
200# echo $$ > /cgroups/0/tasks
201 202Since now we're in the 0 cgroup,
203We can alter the memory limit:
204# echo 4M > /cgroups/0/memory.limit_in_bytes
205 206NOTE: We can use a suffix (k, K, m, M, g or G) to indicate values in kilo,
207mega or gigabytes.
208 209# cat /cgroups/0/memory.limit_in_bytes
2104194304
211 212NOTE: The interface has now changed to display the usage in bytes
213instead of pages
214 215We can check the usage:
216# cat /cgroups/0/memory.usage_in_bytes
2171216512
218 219A successful write to this file does not guarantee a successful set of
220this limit to the value written into the file. This can be due to a
221number of factors, such as rounding up to page boundaries or the total
222availability of memory on the system. The user is required to re-read
223this file after a write to guarantee the value committed by the kernel.
224 225# echo 1 > memory.limit_in_bytes
226# cat memory.limit_in_bytes
2274096
228 229The memory.failcnt field gives the number of times that the cgroup limit was
230exceeded.
231 232The memory.stat file gives accounting information. Now, the number of
233caches, RSS and Active pages/Inactive pages are shown.
234 2354. Testing
236 237Balbir posted lmbench, AIM9, LTP and vmmstress results [10] and [11].
238Apart from that v6 has been tested with several applications and regular
239daily use. The controller has also been tested on the PPC64, x86_64 and
240UML platforms.
241 2424.1 Troubleshooting
243 244Sometimes a user might find that the application under a cgroup is
245terminated. There are several causes for this:
246 2471. The cgroup limit is too low (just too low to do anything useful)
2482. The user is using anonymous memory and swap is turned off or too low
249 250A sync followed by echo 1 > /proc/sys/vm/drop_caches will help get rid of
251some of the pages cached in the cgroup (page cache pages).
252 2534.2 Task migration
254 255When a task migrates from one cgroup to another, it's charge is not
256carried forward. The pages allocated from the original cgroup still
257remain charged to it, the charge is dropped when the page is freed or
258reclaimed.
259 2604.3 Removing a cgroup
261 262A cgroup can be removed by rmdir, but as discussed in sections 4.1 and 4.2, a
263cgroup might have some charge associated with it, even though all
264tasks have migrated away from it.
265Such charges are freed(at default) or moved to its parent. When moved,
266both of RSS and CACHES are moved to parent.
267If both of them are busy, rmdir() returns -EBUSY. See 5.1 Also.
268 269Charges recorded in swap information is not updated at removal of cgroup.
270Recorded information is discarded and a cgroup which uses swap (swapcache)
271will be charged as a new owner of it.
272 273 2745. Misc. interfaces.
275 2765.1 force_empty
277 memory.force_empty interface is provided to make cgroup's memory usage empty.
278 You can use this interface only when the cgroup has no tasks.
279 When writing anything to this
280 281 # echo 0 > memory.force_empty
282 283 Almost all pages tracked by this memcg will be unmapped and freed. Some of
284 pages cannot be freed because it's locked or in-use. Such pages are moved
285 to parent and this cgroup will be empty. But this may return -EBUSY in
286 some too busy case.
287 288 Typical use case of this interface is that calling this before rmdir().
289 Because rmdir() moves all pages to parent, some out-of-use page caches can be
290 moved to the parent. If you want to avoid that, force_empty will be useful.
291 2925.2 stat file
293 memory.stat file includes following statistics (now)
294 cache - # of pages from page-cache and shmem.
295 rss - # of pages from anonymous memory.
296 pgpgin - # of event of charging
297 pgpgout - # of event of uncharging
298 active_anon - # of pages on active lru of anon, shmem.
299 inactive_anon - # of pages on active lru of anon, shmem
300 active_file - # of pages on active lru of file-cache
301 inactive_file - # of pages on inactive lru of file cache
302 unevictable - # of pages cannot be reclaimed.(mlocked etc)
303 304 Below is depend on CONFIG_DEBUG_VM.
305 inactive_ratio - VM inernal parameter. (see mm/page_alloc.c)
306 recent_rotated_anon - VM internal parameter. (see mm/vmscan.c)
307 recent_rotated_file - VM internal parameter. (see mm/vmscan.c)
308 recent_scanned_anon - VM internal parameter. (see mm/vmscan.c)
309 recent_scanned_file - VM internal parameter. (see mm/vmscan.c)
310 311 Memo:
312 recent_rotated means recent frequency of lru rotation.
313 recent_scanned means recent # of scans to lru.
314 showing for better debug please see the code for meanings.
315 316 3175.3 swappiness
318 Similar to /proc/sys/vm/swappiness, but affecting a hierarchy of groups only.
319 320 Following cgroup's swapiness can't be changed.
321 - root cgroup (uses /proc/sys/vm/swappiness).
322 - a cgroup which uses hierarchy and it has child cgroup.
323 - a cgroup which uses hierarchy and not the root of hierarchy.
324 325 3266. Hierarchy support
327 328The memory controller supports a deep hierarchy and hierarchical accounting.
329The hierarchy is created by creating the appropriate cgroups in the
330cgroup filesystem. Consider for example, the following cgroup filesystem
331hierarchy
332 333 root
334 / | \
335 / | \
336 a b c
337 | \
338 | \
339 d e
340 341In the diagram above, with hierarchical accounting enabled, all memory
342usage of e, is accounted to its ancestors up until the root (i.e, c and root),
343that has memory.use_hierarchy enabled. If one of the ancestors goes over its
344limit, the reclaim algorithm reclaims from the tasks in the ancestor and the
345children of the ancestor.
346 3476.1 Enabling hierarchical accounting and reclaim
348 349The memory controller by default disables the hierarchy feature. Support
350can be enabled by writing 1 to memory.use_hierarchy file of the root cgroup
351 352# echo 1 > memory.use_hierarchy
353 354The feature can be disabled by
355 356# echo 0 > memory.use_hierarchy
357 358NOTE1: Enabling/disabling will fail if the cgroup already has other
359cgroups created below it.
360 361NOTE2: This feature can be enabled/disabled per subtree.
362 3637. TODO
364 3651. Add support for accounting huge pages (as a separate controller)
3662. Make per-cgroup scanner reclaim not-shared pages first
3673. Teach controller to account for shared-pages
3684. Start reclamation in the background when the limit is
369 not yet hit but the usage is getting closer
370 371Summary
372 373Overall, the memory controller has been a stable controller and has been
374commented and discussed quite extensively in the community.
375 376References
377 3781. Singh, Balbir. RFC: Memory Controller, http://lwn.net/Articles/206697/ 3792. Singh, Balbir. Memory Controller (RSS Control),
380http://lwn.net/Articles/222762/ 3813. Emelianov, Pavel. Resource controllers based on process cgroups
382http://lkml.org/lkml/2007/3/6/198 3834. Emelianov, Pavel. RSS controller based on process cgroups (v2)
384http://lkml.org/lkml/2007/4/9/78 3855. Emelianov, Pavel. RSS controller based on process cgroups (v3)
386http://lkml.org/lkml/2007/5/30/244 3876. Menage, Paul. Control Groups v10, http://lwn.net/Articles/236032/ 3887. Vaidyanathan, Srinivasan, Control Groups: Pagecache accounting and control
389 subsystem (v3), http://lwn.net/Articles/235534/ 3908. Singh, Balbir. RSS controller v2 test results (lmbench),
391http://lkml.org/lkml/2007/5/17/232 3929. Singh, Balbir. RSS controller v2 AIM9 results
393http://lkml.org/lkml/2007/5/18/1 39410. Singh, Balbir. Memory controller v6 test results,
395http://lkml.org/lkml/2007/8/19/36 39611. Singh, Balbir. Memory controller introduction (v6),
397http://lkml.org/lkml/2007/8/17/69 39812. Corbet, Jonathan, Controlling memory use in cgroups,
399http://lwn.net/Articles/243795/ 400